Generating multi-level high-speed electronic signals is difficult. Typically, integrated circuits such as digital-to-analog converters (DACs) are used to perform this task. However, present day high-speed DACs cannot operate beyond ˜10-GHz bandwidth. Obtaining multi-level electronic signals using a passive combination of multiple signal streams, with binary modulation in each, is also very challenging because of the unavailability of high-quality broadband electronic components.
To overcome this problem, high-speed binary (digital) signals can be combined using broadband passive combiners after precise delay and amplitude adjustments. In this manner, binary signals can be generated at faster speeds. However, broadband combiners and delay elements that are required to accomplish this task are difficult to fabricate, and therefore expensive, because of stringent impedance matching requirements. Small amounts of impedance mismatches can result in signal reflections that can severely degrade the quality of the generated signal, and perhaps make it useless.